|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
7th Aug 2017, 12:37 pm | #21 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,082
|
Re: Replacement rectifier.
Quote:
The reason? It won't make a scrap of difference normally, but if the rectifier ever fails, the resistor will limit the current and hopefully burn-out before the transformer does. Whereas if the resistor is on the DC side, the failed rectifier will be straight across the transformer, which is then almost guaranteed to burn out. |
|
7th Aug 2017, 12:53 pm | #22 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,395
|
Re: Replacement rectifier.
My thinking, too- a conservatively-rated silicon bridge should be reliable but a common failure mode is shorted junctions. It's possible to find power film resistors (2,3 etc. W) that are designed to go fssst, smoke, O/C without drama or fire hazard under over-load.
|
7th Aug 2017, 1:27 pm | #23 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
|
Re: Replacement rectifier.
Sorry I misread the 'off' as of. Often happens with predictive text throwing in extra letters or just typing on a touch screen adding more letters than needed.
Yes it could be the screens connected together to the HT. |
7th Aug 2017, 1:59 pm | #24 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
The pin no on the 4x EL84 is no 9, the amp model no 36196-A, used in several models. The rockola wallmount 430 1484.
|
8th Aug 2017, 9:12 am | #25 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
the amp is from model rockola 430 serial no 36196-A, the valve pin no is 9.
the selenim rec is still in place so i applied 240 v to check the DC output which is 198v, the silconn rec has output of 216v i don't know if this info is useful or is this where the resistor needed |
8th Aug 2017, 10:09 am | #26 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Replacement rectifier.
198V sounds plausible for a selenium rectifier in good condition given that silicon gives you 216V. It probably doesn't need changing.
|
8th Aug 2017, 10:18 am | #27 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
|
Re: Replacement rectifier.
Just found a Rock-Ola circuit with EL84 outputs which shows the HT line as 310v going to pin 9. Fed from a transformer winding to a bridge rectifier but can't make out the voltage shown (could be 235) - very poor photocopy. It may not be the same amp as yours as it has no model numbers on the diagram. Most of their amps seem to use 6L6 or 6973 output valves.
Last edited by vidjoman; 8th Aug 2017 at 10:20 am. Reason: Added info |
8th Aug 2017, 11:01 am | #28 | |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,983
|
Re: Replacement rectifier.
Quote:
Provided they are not running hot and smelling of garlic (Selenium oxide), in which case take the unit outside because of toxicity, and also the house can stink for many days. Craig |
|
8th Aug 2017, 11:04 am | #29 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
does the difference in output voltages create problems in the amp ? i may do as Paul suggests and reinstate the selenium ,is there any way to check it, i have placed an attch in the management section of the amp schematic but not sure how to attch here
|
8th Aug 2017, 11:07 am | #30 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Replacement rectifier.
The small increase in HT probably doesn't matter, but it will largely disappear if you fit the resistor as suggested.
Selenium rectifiers of this type normally fail gradually over several years, dropping more and more voltage and running hot. Occasionally they can fail catastrophically emitting noxious and extremely smelly gas, but this is very rare with audio equipment. |
8th Aug 2017, 11:13 am | #31 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,983
|
Re: Replacement rectifier.
I would be surprised if the additional ten volts or so would be a problem. That is about ten volts in about 300, or 3%. Mains voltage is 230V nominal +10% - 6% and so the selenium voltage drop is swamped by the allowable variation in mains voltage.
There is a good write up about selenium rectifiers, and replacement strategy, here http://yarchive.net/electr/selenium_rectifiers.html Craig |
8th Aug 2017, 11:36 am | #32 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
if i decide on the silicon rec and place a resistor in series with + dc output say 150 ohms what wattage should the resistor be,
|
8th Aug 2017, 11:49 am | #33 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Replacement rectifier.
3W or 5W should be fine. 2W will probably be OK too if you have one to hand.
|
8th Aug 2017, 12:22 pm | #34 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
thanks to Paul and all for help and info, it must be hard trying to explain to
someone with no electronic savvy, cheers |
8th Aug 2017, 12:45 pm | #35 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Replacement rectifier.
The OP said the amp is a 36196-A, the schematic that the OP posted is for a 41710-A.
Looking at another schematic, for the Rockola 434, it's output stage looks very similar to the one in the schematic that the OP posted, the cathode voltage and resistors for the EL84's are shown as the same so far as I can make out (10v and 130 ohms) The 434 schematic shows the HT winding as 235v and the HT as 315v, the OP has reported the measured HT as 198v (selenium) and 216v (silicon): http://arcarc.xmission.com/PDF_Jukeb...atics/434a.pdf ? Lawrence. |
8th Aug 2017, 1:00 pm | #36 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,947
|
Re: Replacement rectifier.
I did think the voltage seemed a bit low for an amp, but as I said I don't have any experience with juke boxes. The voltage from the silicon bridge does correspond to the selenium voltage though, so if there's a fault it's somewhere else.
|
8th Aug 2017, 1:15 pm | #37 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
hi Lawrence
amps are the same different numbers for two models and years but same schematics there are 3 different model no's but same amp |
8th Aug 2017, 1:20 pm | #38 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
Re: Replacement rectifier.
According to your measurements it looks like the HT's low then, measure the AC voltage across the mains transformers HT winding, then across the AC input at the bridge.
Lawrence. |
8th Aug 2017, 2:26 pm | #39 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
|
Re: Replacement rectifier.
The schematic for the 434a as linked by ms660 is the same as the copy I have which gives 315 as the HT.
|
8th Aug 2017, 3:14 pm | #40 |
Retired Dormant Member
Join Date: Aug 2017
Location: Morecambe, Lancashire, UK.
Posts: 13
|
Re: Replacement rectifier.
the output on the trani is 244v same at the rectifier ac legs ,,attch
schematic of pwer supply + amp |